electromotive force
◆Discharge performance of energy storage batteries
When the external circuit is open, meaning no current flows through the battery, the potential difference between the positive and negative electrodes is defined as the battery's electromotive force (EMF), usually denoted by the symbol E. The value of the EMF is one of the indicators reflecting the amount of electrical energy that the battery system can output. According to thermodynamic principles, we have...
In the formula, AG represents the change in Gibbs free energy; n represents the electron transfer number; E represents the cell electromotive force; and F represents the Faraday constant.
Equation (1.2) shows that the magnitude of the battery electromotive force mainly depends on the inherent properties of the substances involved in the chemical reaction, the reaction conditions (such as temperature) during battery operation, and the activity of the reactants and products, and is not affected by the battery geometry or size.
Open circuit voltage
The open-circuit voltage of a battery refers to the potential difference between the positive and negative terminals when the external circuit is disconnected from the battery. It's important to note that because the positive and negative terminals may not have reached thermodynamic equilibrium in the electrolyte solution,the open-circuit voltage is usually slightly lower than its electromotive force (EMF). The EMF is a theoretical value calculated based on thermodynamic formulas, while the open-circuit voltage is an actual value obtained through direct experimental measurement; the two are very close in value. To accurately determine the open-circuit voltage, no current should flow through the measuring instrument during the measurement process; a high-resistance voltmeter is generally used for this test.
Furthermore, the concept of nominal voltage is also used in production research. Nominal voltage is an appropriate approximation of the voltage of a battery, also known as rated voltage, and is used to identify the battery type. For example, the open-circuit voltage of a lead-acid battery is close to 2.1V, and its nominal voltage is set at 2.0V; the nominal voltage of a zinc-manganese battery is 1.5V; and the nominal voltage of a cadmium-nickel battery and a nickel-metal hydride battery is 1.2V.
Internal resistance
The internal resistance of a battery, commonly referred to as internal resistance (Rinternal), refers to the resistance encountered when current flows through the battery. This internal resistance mainly consists of two parts: one is the ohmic internal resistance caused by the properties of the material itself; the other is the additional polarization internal resistance generated by the polarization phenomenon on the electrode surface during the electrochemical reaction.
The ohmic resistance (R0) is affected by the electrolyte characteristics, membrane properties, and electrode material. The ohmic resistance of an electrolyte solution is closely related to its specific composition, concentration level, and ambient temperature. Generally, the electrolyte concentration for batteries is selected within the range of highest conductivity. The resistance caused by the membrane micropores to the migration of electrolyte ions is called membrane resistance, i.e., the resistance encountered by ions as they pass through the membrane micropores. The ohmic resistance of the membrane is related to factors such as the type of electrolyte, the membrane material, porosity, and the degree of pore tortuosity. The solid-phase resistance on the electrodes includes the resistance of the active material particles themselves, the contact resistance between particles, the contact resistance between the active material and the conductive framework, and the sum of the resistances of the conductive framework, conductive busbars, and terminals. During discharge, the composition and morphology of the active material may change, resulting in significant changes in resistance. To reduce solid-phase resistance, conductive components, such as acetylene black-graphite, are often added to the active material to increase its conductivity. The ohmic internal resistance of a battery is also related to factors such as its size, assembly, and structure. The more compact the assembly and the smaller the electrode spacing, the lower the ohmic internal resistance.
Polarization resistance (R) refers to the internal resistance generated in a chemical power source due to polarization during electrochemical reactions at the positive and negative electrodes. Polarization resistance includes the sum of resistances caused by electrochemical polarization and concentration polarization. Its magnitude is influenced by the properties of the active materials, electrode structure, and battery manufacturing process, and is particularly closely related to the battery's operating state. Therefore, the polarization resistance changes accordingly with variations in discharge mode and discharge time.
Operating voltage
The operating voltage of a battery, also known as the load voltage or discharge voltage, refers to the potential difference between the positive and negative terminals of the battery when current flows through an external circuit. When current flows through the battery, in order to overcome the resistance caused by polarization resistance and ohmic resistance, the actual measured operating voltage value is always lower than the open-circuit voltage under no-load conditions.
As can be seen from equation (1.3), the greater the internal resistance of the battery, the lower the operating voltage of the battery, and the smaller the actual energy output. Obviously, the internal resistance of the battery should be as small as possible.